The present invention relates to a positioning apparatus such as may be used for aligning masks for processing of semiconductor devices.
In order to carry out a positioning operation over a wide range with high accuracy, it is necessary to use a precision position measuring device and a shifting mechanism having a large stroke which is capable of moving an extremely short distance. A piezoelectric worm type shifting mechanism (hereinafter referred to as "a piezo motor" when applicable) is a known mechanism of the latter type. The piezo motor can move an extremely short distance of the order of several angstroms while having a maximum stroke of several tens of millimeters. Such a piezo motor, as shown in FIGS. 1(a) and 1(b), includes cylindrical piezoelectric elements 11, 12 and 13. Upon application of a voltage to the piezoelectric elements 11 and 13, the piezoelectric elements 11 and 13 contract radially with respect to the longitudinal axes of the cylinders. On the other hand, upon application of a voltage to the piezoelectric elements 12, the piezoelectric elements 12 expand longitudinally.
The operating principles of the piezo motor is as follows. To move the piezo motor to the right as viewed in FIG. 1(a), first no voltage is applied to the clamping piezoelectric elements 11 so that the elements 11 are in a position in abutment with an external cylindrical body 15. Then, a voltage is applied to the clamping piezoelectric elements 13 causing them to contract and thus clamp a moving bar 14. Thereafter, a voltage is applied to the piezoelectric elements 12 and the voltage thus applied is increased stepwise and gradually so that the elements 12 expand longitudinally in a motor which moves the moving bar 14 to the right. When the voltage applied to the piezoelectric elements 12 has reached its allowable maximum value, the polarity of the voltage applied to the clamping piezoelectric elements 11 and 13 is reversed so that the piezoelectric elements 11 clamp the moving bar 14 as shown in FIG. 1(b). Under this condition, the voltage applied to the piezoelectric elements 12 is decreased stepwise so that the elements 12 contract longitudinally to therefore again move the moving bar 14 to the right. In a similar fashion, the piezo motor can be moved to the left by suitably controlling the voltages applied to the piezoelectric elements 11, 12 and 13.
A method of performing a positioning operation using the above-described piezo motor will be described with reference to FIG. 2.
FIG. 2 is a block diagram showing a conventional precise positioning apparatus using a piezo motor. A platform 22 is driven by a piezo motor 21 and the position of the platform 22 is measured with a position measuring device 23. The output data of the position measuring device 23 is applied to a microprocessor 25 which calculates a number of pulses corresponding to the distance through which the platform 22 should be moved to arrive at the desired position. A corresponding number of pulses are generated by a pulse generating circuit 27. The pulses are inputted to a counter 28 and then the output of the counter 28 applied to a D/A converter 29 to be subjected to D/A conversion to thereby generate a stair step waveform signal. The stair step waveform signal is applied through a control section 24 to drive the piezo motor 21.
For the discussion which follows, for purposes of illustration it is assumed that the piezo motor is so designed that the platform moves 0.006 .mu.m per step of the stair step waveform, for instance. It is further assumed that when the position of the platform 22 is measured at a point 41 in FIG. 4 by the position measuring device 23, the platform 22 is 0.06 .mu.m distant from the desired position. In this case, the microprocessor 25 carried out an operation, namely the division of the error distance 0.06 .mu.m by the incremental step length 0.006 .mu.m, and applies a stair step voltage corresponding to ten pulses to the piezo motor control section 24 as a result of which the piezo motor is driven by ten steps to move the platform 22. The position of the platform 22 is again measured at a point 42 indicated in FIG. 4. If, at this time, the platform 22 has been moved beyond the desired position by a distance of, for example, 0.024 .mu.m, the driving direction of the piezo motor is reversed for four steps. That is, the position of the platform 22 is repeatedly measured and the piezo motor repeatedly driven according to the measurement results until the position of the platform 22 is within an allowable position range around the desired position.
The above-described conventional positioning method is disadvantageous in that in order to change the voltage stepwise, for instance, from the value at the point 41 to the value at the point 42 in FIG. 4, a period of time is required which is the product of the period of one step and the required number of steps, ten in the above-described case. That is, if the number of steps is increased, the period of time required for completion of the positioning operation is proportionally increased.